1.1. TECHNICAL FIELD
The present invention relates to compositions useful as an alkaline developable liquid photoimageable solder resist ink and the like, having excellent adhesion, heat resistance, and electrical insulating properties in an atmosphere having a high humidity. Moreover, the compositions of the invention are additionally developable in an aqueous alkaline system in contrast to related materials in the prior art.
1.2. DESCRIPTION OF RELATED ART
Solder resist inks have been employed in the prior art to prevent solder from spreading over portions of a printed circuit board where it is not required to protect the circuit. Various properties, such as adhesion, heat resistance, moisture resistance and chemical resistance are thus required for these solder resist inks.
Traditionally, solder imaging of printed circuit boards has been achieved by a process known as screen printing. With the growing complexity of printed circuit board design however, particularly in the professional markets, solder masks have had to cope with increasing demands on print definition. The screen process is thus being pushed to the limit of its capabilities.
To solve the problem described above, a so-called solvent developable photo resist has been developed. This type of photo resist has a secondary problem, however, in that it cannot be employed due to environmental and health problems caused by the large amount of solvent consumed in the process of its development.
Recently, so-called "dry film" photo resists, which do not employ an organic solvent, and so-called "liquid alkaline developable" photo resists have been developed. However, when dry film photo resists are employed, problems have occurred in the boundary area between the resist and the substrate during a subsequent thermocompression bonding process. In addition, the resultant photo resist lacks adhesion and heat resistance. Furthermore, the higher cost involved hampers wide use of this system.
Moreover, although many kinds of liquid alkaline developable photo resists have been developed, one which satisfies the requirements for all of the properties discussed above, such as adhesion, heat resistance, moisture resistance and developability in an alkaline system has not yet been produced. For example, in instances where an epoxy acrylate resin is utilized as the photo-curable resin, the material has been found to be hygroscopic and it thereby sacrifices its electrical insulating ability due to the many hydroxyl groups which remain in the resin. Further, it is necessary to add an alkaline-soluble binder into the photo-curable resin to produce an alkaline developable resin, and when the alkaline-soluble binder is contained in the photo-curable resin, the resultant system provides poor adhesion and heat resistance. In addition, to improve the ability of the photo resist to develop in an alkaline solution, an epoxy-acrylate resin, esterified with polybasic anhydride has been developed, but the resin thus produced has insufficient adhesion.
In order to improve its adhesive and non-hygroscopic properties, the resin described above has also been modified, as known in the prior art, with a urethane resin. However, urethane bonds in the resin structure reduce the solubility of the resin to a solvent which is an essential component in the process of preparing an ink composition. This results in the dilution of the effective components. Consequently, the material's chemical resistance and alkaline developable properties are deteriorated. Also, even though an alkaline-soluble binder is added to the resist, the ability of the material to be developed in an alkaline system is not improved.